1. Field of the Invention
The present invention relates to an unit which is provided with a communications interface circuit, such as RS 232C, more particularly to an improvement of a data communications system comprising a data terminal equipment (hereinafter simply referred as DTE) and a data communications equipment (hereinafter simply referred as DCE), that is, a modem.
2. Description of the Prior Art
In recent years, data communication, in which data is received or sent among data terminal equipments (hereinafter referred to DTE) over telephone lines, has rapidly prevailed throughout the world.
With the development of this data communication, a communications interface has been normalized in order to establish a standardization of a data transmission control among DTEs. For example, RS 232C interface was standardized by EIA (Electronics Industries Association) as an interface to link between a DTE and a modem. Since then, RS 232C interface has been put into practice as the international standard.
Accordingly, the standardized interface circuit allowed an independent DTE to link with another DTE, realizing the data communication.
However, in the conventional communications interface, such as RS 232C interface, a load output from the driver circuit is specified to be from 5 V to 15 V as shown in Table 1.
This means that the driver circuit of each communications interface transmits a signal with a relatively large amount of electric power taking into account that an input signal should be secured at the receiver circuit maintaining the specified voltage even when the signal is transmitted through a cable with a maximum length. For example, in the case of RS 232C interface, a voltage ranging from 5 V to 15 V is to be secured at the receiver after the signal passed through the cable of 15 meters.
Moreover, in the conventional technique, a data communications system, with which a communications interface circuit to realize a data communication is provided, is driven by its dedicated power supply circuit.
TABLE 1 ______________________________________ Specifications of RS 232 C ______________________________________ Operation Unbalanced Power Supply Voltage .+-.12 V Driver Circuit Maximum cable extension 15 m Maximum data rate 10 kBaud No-load output voltage .ltoreq.25 V Load output voltage 5.about. 15 V Output resistance value .gtoreq.300 .OMEGA. when power is off. Output current value when .ltoreq.500 mA a circuit is shorted. Through output rate .ltoreq.30 V/.mu.S Receiver Circuit Input resistance 3 k .about. 7 k.OMEGA. Threshold -3.about. +3 V Maximum input voltage .+-.25 V ______________________________________
As has been described above, there has been a problem that the independent power supply is necessary for the driver, for example, the DTE and the receiver circuit, such as the DCE, respectively in the conventional data communications system, which a communications interface circuit is built in. Therefore, the communication system was hard to be miniaturized.
Another fact is that power has been wastefully dissipated in the receiver circuit of the DCE because the DCE, which the communications interface circuit is provided with, has usually been arranged in proximity to the DTE, such as a personal computer, linking them with a short cable, so that the signal loss can result only in small amount.